CNBC's Bob Pisani recently got an exclusive inside look at the HSBC gold vaults in London, where the gold for the SPDR Gold Trust (GLD) is stored. One section of the vault contains gold allocated specifically to the GLD, but there are other clients that store their gold in other parts of the vault as well.

HSBC houses the vault at an undisclosed location in London. It is the largest private gold reserve in the world, with more than $70 billion, or 40 million ounces worth of gold in one place.

As one might expect, the bullion is very closely regulated. Each bar weigh about 400 ounces, and all required to be at least 99.5 percent pure gold.

HSBC's Global head of precious metals, Jeremy Charles says that standard is critical: "The quality of the bars is assured because there is something called the London Gold Delivery list. That list is overseen by the London Bullion Market Association. And to be on that list, you have to have passed a very stringent set of rules."

According to Charles, gold is moved into and out of the vault regularly, depending on how many shares of SPDR exchange traded product are created or redeemed. Today there is six times as much gold as there was in 2005 — all due to increased demand for gold —and higher prices.